Avoid using HTML in translation strings (wp-includes/customize/class-wp-customize-custom-css-setting.php)

[SergeyBiryukov]

In the past, we've made an effort to remove <code> tags from translatable strings, as they generally do not need to be translated.

[38829] introduced a bunch of new strings with <code> tags in WP_Customize_Custom_CSS_Setting class for displaying error messages when validating custom CSS rules.

I've just stumbled upon a string where <code>content: "";</code> was translated literally, which means we should remove non-translatable parts from these strings.

[SergeyBiryukov]

In 40067:

I18N: Remove <code> tags from translatable strings in wp-includes/customize/class-wp-customize-custom-css-setting.php.

Fixes #39898.

[swissspidy]

In 40072:

I18N: Fix broken sprintf() after [40067].

See #39898.

[swissspidy]

In 40073:

I18N: Update translators comment after [40072].

Props ocean90.
See #39898.